1. Field of the Invention
The invention relates to a method for synthesis and process inhibition of isobornyl (meth)acrylate, which is widely used as a monomer for manufacture of, for example, varnish binding agents.
2. Discussion of the Background
Conventionally, camphene is reacted with methacrylic acid in the presence of an acid catalyst to yield isobornyl (meth)acrylate. For example, German Patent DE 44 19 686 (Hoechst AG) describes a process for the synthesis of terpene esters by reacting camphene and a low molecular weight carboxylic acid using an acid ion exchanger as a catalyst. The reagents are passed in ascending flow through a column-type reactor containing a bed of acid ion exchanger. However, a disadvantage in this process is the loss of activity of the catalyst after several reaction cycles. Attempts to compensate for this loss by raising the temperature have the disadvantage of increasing the polymerization of the unsaturated carboxylic acids on the catalyst, leading in time to irreversible deactivation of the catalyst. The process is therefore not suitable for large-scale industrial applications, which require long catalyst lives.
European Patent EP 718 271 (Elf Atochem) also describes the synthesis of isobornyl methacrylate or acrylate by reacting camphene with acrylic acid or (meth)acrylic acid in the presence of an acid ion exchange catalyst. Although ion exchange catalysis seems elegant at first sight, in industrial practice it has the disadvantage that the ion exchanger loses activity after repeated use, since it becomes coated with polymer despite good process stabilization. Regenerative cleaning and activation, for example by washing with acid or a suitable solvent for drying such as acetone, does not restore the original activity level. Accordingly, the catalyst must be replaced after a few cycles of use, unless poorer conversion is acceptable.
According to the method of Japanese Laid-Open Application JP-0S 54-126293, only 10% conversion is achieved. In view of the relatively expensive starting materials, such low conversion during the reaction is economically impractical. Satisfactory conversions can be achieved with 75% sulfuric acid when the reaction mixture is very strongly inhibited. The benefit of inhibition of the reaction mixture is offset by disadvantages in regard to the product, which becomes correspondingly difficult to polymerize.
A further alternative to the synthesis of isobornyl (meth)acrylate is transesterification of methyl methacrylate starting from isoborneol. The process of transesterification of methyl methacrylate to obtain esters of (meth)acrylic acid is known.
German Patent DE 196 02 035 (Rohm GmbH) describes the transesterification of (meth)acrylic acid esters of short-chain alcohols with long-chain alcohols (C1 to C28) to obtain (meth)acrylic acid esters of long-chain alcohols in the presence of a transesterification catalyst comprising Ca(OH)2 or a mixture of Ca(OH)2 and LiCl.
Furthermore, German Patent DE 19 54 709, Rxc3x6hm GmbH describes a catalytic process for transesterification of (meth)acrylic acid esters using at most 250 ppm of Ca(OH)2 as a catalyst.
German Patent DE 195 45 870 (Rxc3x6hm GmbH) discloses a transesterification of lower (meth)acrylic acid esters to (meth)acrylic acid esters of carbohydrates having at least one free hydroxyl group. A mixture of alkali metal carbonate and a quaternary ammonium salt is used as the transesterification catalyst.
German Patent DE 44 011 32 (Rxc3x6hm GmbH) describes the transesterification of (meth)acrylic acid esters with alcohols which contain one or more esterifiable hydroxyl groups in the presence of a mixed catalyst comprising 5 to 95 mol % of diorganyltin oxide and 95 to 5 mol % of diorganyltin dihalides. The mixed catalyst is used in proportions of between 0.01 and 10 wt % based on the weight of the reaction mixture.
German Patent DE 43 01 673 (Rxc3x6hm GmbH) describes the use of a lead compound as the transesterification catalyst. Dialkyltin oxide alone as the transesterification catalyst has also already been described (German Patent DE 4010532, Rxc3x6hm GmbH).
The esterification of an aqueous (meth)acrylic acid (water content: 5 to 60 wt %) with an alkanol to obtain the corresponding (meth)acrylic acid esters is disclosed in German Patent DE 31 46 191 (Rxc3x6hm GmbH). In this process the aqueous (meth)acrylic acid is passed into a boiling esterification mixture containing H2SO4 or an organic sulfonic acid and the resulting ester is distilled off.
U.S. Pat. No. 3,087,962 (Rohm and Haas) describes the synthesis of esters of acrylic acid or of methacrylic acid by esterification of acrylic acid or methacrylic acid under catalysis by H2SO4 and BF3.
According to the known transesterification procedures, isobornyl methacrylate is obtained in high purity and good yield from isoborneol and (meth)acrylic acid methyl ester in a smooth reaction. Isoborneol is not commercially available. However, isobornyl acetate, which is also used as a scent, can be obtained inexpensively in the market. Tests have shown that isobornyl acetate cannot be reacted with either methyl (meth)acrylate or (meth)acrylic acid to obtain isobornyl (meth)acrylate by means of the standard procedure of adding a known transesterification catalyst.
It is an objective of the present invention to find, preferably starting from inexpensive raw materials, an industrially feasible synthesis of isobornyl (meth)acrylate which can be performed without the use of complex mixed catalysts and which does not require excessive stabilization of the reaction mixture.
These and other objects are achieved according to the invention, the first embodiment of which includes a process for the synthesis of isobornyl (meth)acrylate, comprising:
reacting isobornyl acetate with a compound having a hydroxyl group in a reaction mixture to obtain isoborneol;
reacting said isoborneol with a (meth)acrylic acid ester thereby obtaining isobornyl (meth)acrylate.
It has now been found that isobornyl acetate, which is used as a scent and which is commercially available, can be converted to isobornyl methacrylate. However, isobornyl acetate cannot be reacted with either methyl (meth)acrylate or (meth)acrylic acid to obtain isobornyl (meth)acrylate by means of the known procedures of adding a transesterification catalyst.
A simple isobornyl (meth)acrylate synthesis is successful if isobornyl acetate 1 is reacted in a first stage with methanol 2 and a transesterification catalyst in a one-pot process to obtain isoborneol 3 and methyl acetate 4. This reaction step is followed by addition of methyl (meth)acrylate 5 in the presence of the same catalyst as in stage 1, whereby isobornyl (meth)acrylate 6 is produced. The intermediate products are not isolated. The raw ester is already formed in very pure condition. Purification by distillation is not necessary. However, the reaction can be followed by an additional purification step.
Lead compounds, zinc compounds, alkaline earth metal oxides or alkaline earth metal hydroxides, such as CaO, Ca(OH)2, MgO, Mg(OH)2 or mixtures of the aforesaid compounds, as well as alkali metal hydroxides, alkoxides and LiCl can be used as transesterification catalysts. Mixtures of the aforesaid alkaline earth metal compounds and the Li salts can be used as transesterification catalysts. A mixture of 75% by weight of calcium oxide and 25% by weight of lithium hydroxide per 100% by weight of the catalyst mixture is preferred. The catalyst mixture can include 50, 55, 60, 65, 70, 75, 80, 85, 90 and 95% by weight of calcium oxide including all values therebetween based on the weight of the catalyst mixture. Furthermore, the catalyst mixture can include 5, 10, 15, 20, 25, 30, 35, 40, 45 and 50% by weight of lithium hydroxide including all values and subvalues therebetween based on the total weight of the catalyst mixture.
The proportion of catalyst or mixture of catalysts ranges between 0.01% by weight and 10% by weight based on the weight of the reaction mixture at the beginning of the reaction. A proportion of 0.5% by weight to 5% by weight is preferred, and a proportion of 1% by weight to 3% by weight is especially preferred. The proportion of catalyst or catalyst mixture can include all values and subvalues therebetween especially including 0.02, 0.05, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0 and 9.5% by weight based on the weight of the reaction mixture at the beginning of the reaction.
Isobornyl (meth)acrylate can be obtained with a purity of at least 85%, preferably at least 90%, more preferably at least 95% and most preferably at least 99%. The purity includes all values and subvalues therebetween, especially including 86, 88, 90, 92, 94, 96 and 98%.
Preferred hydroxyl compounds are monohydric alcohols, dihydric alcohols and polyfunctional alcohols. Preferably, ethanol or propanol are used as hydroxyl compounds. Most preferably, methanol is used as a compound having a hydroxyl group.
Hydroquinone monomethyl ether, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate and 4-hydroxy-2,2,6,6-tetramethylpiperidyl-N-oxyl are preferably used as stabilizers. 
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.